SHM_OPEN(3) BSD Library Functions Manual SHM_OPEN(3)NAME
shm_open, shm_unlink — shared memory object operations
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/mman.h>
int
shm_open(const char *path, int flags, mode_t mode);
int
shm_unlink(const char *path);
DESCRIPTION
The shm_open() function opens (or optionally creates) a POSIX shared mem‐
ory object named path. The shm_unlink() function removes a shared memory
object named path.
In the DragonFly implementation, POSIX shared memory objects are imple‐
mented as ordinary files. The shm_open() and shm_unlink() act as wrap‐
pers around the open(2) and unlink(2) routines, and path, flags, and mode
arguments are as specified for those functions. The flags argument is
checked to ensure that the access mode specified is not O_WRONLY (which
is not defined for shared memory objects).
In addition, the DragonFly implementation causes mmap() of a descriptor
returned by shm_open() to behave as if the MAP_NOSYNC flag had been spec‐
ified to mmap(2). (It does so by setting a special file flag using
fcntl(2).)
The shm_unlink() function makes no effort to ensure that path refers to a
shared memory object.
RETURN VALUES
If successful, shm_open() returns a non-negative integer; shm_unlink()
returns zero. Both functions return -1 on failure, and set errno to
indicate the error.
COMPATIBILITY
The path argument does not necessarily represent a pathname (although it
does in this and most other implementations). Two processes opening the
same path are guaranteed to access the same shared memory object if and
only if path begins with a slash (‘/’) character.
Only the O_RDONLY, O_RDWR, O_CREAT, O_EXCL, and O_TRUNC flags may be used
in portable programs.
The result of using open(2), read(2), or write(2) on a shared memory
object, or on the descriptor returned by shm_open(), is undefined. It is
also undefined whether the shared memory object itself, or its contents,
persist across reboots.
ERRORS
The shm_open() and shm_unlink() functions can fail with any error defined
for open() and unlink(), respectively. In addition, the following errors
are defined for shm_open():
[EINVAL] The object named by path is not a shared memory object
(i.e., it is not a regular file).
[EINVAL] The flags argument to shm_open() specifies an access
mode of O_WRONLY.
SEE ALSOmmap(2), munmap(2), open(2), unlink(2)STANDARDS
The shm_open() and shm_unlink() functions are believed to conform to IEEE
Std 1003.1b-1993 (“POSIX.1”).
HISTORY
The shm_open() and shm_unlink() functions first appeared in FreeBSD 4.3.
AUTHORS
Garrett A. Wollman ⟨wollman@FreeBSD.org⟩ (C library support and this man‐
ual page)
Matthew Dillon ⟨dillon@FreeBSD.org⟩ (MAP_NOSYNC)
BSD March 24, 2000 BSD
